Epstein-Barr virus (EBV) is a member of the human herpesviruses, all of which are membrane enveloped viruses that utilize multiple glycoproteins to enter cells. More than 90% of the adult population is sero-positive for EBV, which efficiently infects both epithelial and B cells, the latter providing the latency reservoir. Acute EBV infection acquired in adolescence or adulthood can cause infectious mononucleosis, accompanied by a proliferation of B cells. In addition, both cell types can develop tumors that are directly linked to EBV such as nasopharyngeal carcinoma and Burkitt's lymphoma. Other malignancies also connected to EBV include post-transplant lymphoproliferative disorder (PTLD) as well as oral hairy leukoplakia and B cell lymphomas of the central nervous system prevalent in AIDS and immunosuppressed individuals. The medical relevance of EBV makes it an important subject of study, especially understanding the mechanism by which the virus enters its two target cell types.
The minimum requirement for membrane fusion to occur with epithelial cells the coexpression of EBV envelope glycoproteins gH, gL, and gB. EBV additionally requires the gp42 protein for entry into B cells (Haan et al., 2001, Virology 290:106-14; Li et al., 1995. J. Virol. 69:3987-94; and McShane et al., 2004, PNAS USA 101:17474-9, incorporated by reference herein in their entireties). EBV gp42 binds major histocompatibility complex (MHC) class II proteins expressed on B cells to trigger viral-cell membrane fusion. A soluble gp42-Fc chimeric protein can stimulate the entry of a gp42-null virus into B cells and likewise, the addition of baculovirus-produced, soluble gp42 to cells transfected with gH, gL, and gB allows membrane fusion to occur with B cells (Kirschner et al., 2006, J. Virol. 80:9444-54; and Wang et al., 1998, J. Virol. 72:5552-8, incorporated by reference herein in their entireties). However, membrane fusion with epithelial cells is actually inhibited by the presence of gp42 for both virus infection and a cell:cell fusion assay. Increasing levels of inhibition occur as exogenous soluble gp42 is added in a cell:cell fusion assay, beginning in the low nanomolar concentration range.
Nearly all herpesviruses contain a gH/gL complex, which serves an indispensable role in membrane fusion and infection. EBV gH requires the presence of gL in order for EBV gH to fold properly and be transported to the cell membrane, and both EBV gL and the related varicella-zoster virus (VZV) gL proteins function effectively in mediating the folding and expression of EBV gH protein. It has also been established that EBV gH/gL exists as a non-covalently associated heterodimer complex in 1:1 subunit ratio.
EBV and other herpesviruses use a multicomponent system for membrane fusion, which is different from other known viral fusion systems that utilize a single fusion protein. The presence or absence of EBV gp42 appears to act as a switch for virus entry into B cells or epithelial cells, respectively, and its interaction with gH/gL is a fundamental feature in EBV-mediated membrane fusion.
There is a need in the art for a better understanding of the interactions between EBV gH/gL and gp42 in the virus entry mechanism. Variants of gp42 that inhibit EBV-mediated cell fusion may provide new peptide-based drugs for treating and prevented conditions and diseases associated with EBV infection.